7mm White freshwater pearl sterling silver stud earrings

NWT White freshwater pearls on sterling silver studs 7mm come with clear silicone stoppers ( hypoallergenic) Why are some pearl studs more expensive or cheaper than others? Other than the post, which may be costume, steel, silver or gold... it's the quality of the pearl. Getting a pair natural freshwater pearls of nearly similar size and colour adds to the price also.